Cavitation is the formation and implosion or collapse of cavities, or bubbles, in a liquid that are the consequence of forces acting upon the liquid. Cavitation usually occurs when the local pressure within a liquid is reduced to less than the vapor pressure of the liquid.
Many industrial processes rely on various chemical reactions in a liquid medium to achieve a certain end product. Accordingly, manufacturers and others that perform these industrial processes continually seek improvements to these processes so as to improve their efficiency and provide a cost benefit. By way of example, increasing the efficiency in chemical reactions occurring in a liquid medium may result in a decrease in processing time, which may lead to an increase in overall production and decrease in operating costs, and/or a decrease in chemical consumption in the liquid medium for achieving the desired result which may reduce operating costs. These are only exemplary and, depending on the specific application, many other benefits may be gained by improving the efficiency of various chemical reactions.
There are numerous industrial applications that may benefit from improved chemical reactions using cavitation. For example, the use of cavitation in the treatment of contaminated water, e.g., wastewater, is documented. In these cavitation methods, the goal is to generate many fine bubbles, which upon their implosion create intense, but highly localized temperatures and pressures. This energy release then causes dissolution of the water molecules and the creation of free hydroxyl radicals. The potential of these powerful radicals for the beneficial treatment of the water has been well recognized for many years. However, the inefficiencies in the known processes for generating cavitation within a liquid (e.g., ultrasonic, spinning impeller, or jet cavitation) have limited commercial acceptance of cavitation.
It should be noted that in cavitation, the process of generating bubbles is often considered secondary in importance to the subsequent process of collapsing the bubbles, since it is the collapse which produces the effecting high temperatures and pressures. More intense bubble collapse results in higher temperatures and allows more energetic bonds to be broken. This increases the ability of the cavitation process to kill microorganisms and separate particulate matter, such as breaking bonds between or within molecules. Maximum bubble collapse temperatures and pressures are achieved by maximizing bubble collapse velocities.
Preferably, in order to maximize effectiveness, bubbles collapse in rapid succession in a process referred to as cavitation cloud collapse, generating high amplitude/frequency pressure waves which can be used to generate high pressure differentials around and across the bubbles. Cavitation cloud collapse can generate pressures that are orders of magnitude higher than those produced by single bubble collapse. However, this synchronization is difficult to achieve. When an individual bubble collapses under uniform ambient pressure, the collapse rate is restrained due to the inertia of the surrounding fluid. Following collapse, a pressure wave is generated which radiates energy spherically outward from its center. The amplitude of the pressure wave is reduced as a function of the square of the distance from the center of the bubble collapse, such that the amplitude decreases relatively rapidly. If several bubbles collapse in a random manner, pressure waves generated from these collapses impact upon other bubbles in a random manner. The amplitudes of the resulting pressure waves will also vary in a random manner. Due to the random nature of these pressure waves, cavitation cloud collapse will not be initiated, and maximum bubble collapse temperatures and pressures will not be achieved. However, if a uniform, coherent, high frequency pressure wave encounters a cloud of bubbles, then the increase in differential pressure applied to the bubbles will tend to cause the bubbles to collapse in the direction of the pressure wave, such that the energy from the bubble collapses will be added to the original pressure wave, increasing its amplitude. This will tend to increase the amplitude of the individual pressure pulses generated by successive bubble collapses, and will also tend to increase the frequency of these pressure pulses.
When uncontrolled, hydrodynamic cavitation can be very damaging. Studies have shown that cloud collapse can be more violent than collapse of individual bubbles. Typically, the damage is most severe on solid surfaces close to the location of the cloud collapse. Shock waves formed by clouds of cavitation bubbles that implode on or near a metal surface can cause cyclic stress through repeated implosion, resulting in surface fatigue of the metal. As a result, leaks can eventually form in the walls of a device in which cloud cavitation occurs. Therefore, the implosion of cavitation bubble clouds must be controlled in order to harness their energy and reduce or eliminate damage to the surrounding walls.
A wide variety of cavitation generators exist in the prior art. For example, U.S. Pat. No. 4,474,251 to Virgil E. Johnson, Jr. describes an acoustic-hydrodynamic resonator which may utilize an organ-pipe oscillator in conjunction with a Helmholtz resonator chamber to produce a pulsed liquid jet for eroding a solid surface. In one embodiment, liquid is directed through a first orifice and a jet is formed by directing the liquid through a second orifice, and the jet is pulsed by oscillating the pressure of the liquid after it exits the first orifice through hydrodynamic and acoustic interactions. Typically, a Helmholtz chamber is formed between the first and second orifices, wherein the pressure of the liquid is oscillated within the Helmholtz chamber, and a portion of the energy of the high velocity liquid is utilized to pulse the liquid. The cavitation bubble collapse occurs within the discharge stream and not within the device. The device in the '251 patent may be termed a pure fluid device since it is entirely passive and requires no outside energy supply. The energy for its operation comes only from the fluid and it depends on hydrodynamic and acoustic interactions for its operation. In addition, several hydrodynamic cavitation generators have been invented by Oleg V. Kozyuk and others. However, none of these devices adequately generate cavitation bubble cloud collapse, and in particular, capture and utilize residual pressure wave energy to enhance formation and collapse of subsequent cavitation bubble clouds.
It is therefore an object of the present invention to provide a cavitation generator which maximizes the utilization of fluid energy for cavitation bubble formation and which maximizes the temperatures and pressures generated during bubble collapse.